The present invention relates to an image reading apparatus for reading an image on an original, and dimming control method and line sensor layout method therefor.
Conventionally, various image reading apparatuses for forming the image of image information on an original on a plurality of line sensors (solid-state image sensing elements such as CCDs) through an imaging optical system and reading the image as monochromatic or color digital image information on the basis of output signals from the line sensors have been proposed.
FIG. 29 is a schematic view showing principal part of the optical system of a conventional color image reading apparatus.
Referring to FIG. 29, an original (not shown) placed on an original glass table 100 is illuminated with a rod-shaped light source 101. A reflecting shade 102 is used to improve the illumination efficiency. Light from the original illuminated with the rod-shaped light source 101 and reflecting shade 102 is guided to an imaging optical system 104 through mirrors 103-a, 103-b, and 103-c. The imaging optical system 104 forms the image of image information of the original on a solid-state image sensing element (line sensor) 105.
The line sensors 105 comprises three line sensors independently prepared for R, G, and B signals. A light amount sensor 106 detects the light amount of the rod-shaped light source 101. The rod-shaped light source is ON/OFF-controlled on the basis of the output from the light amount sensor 106 such that the rod-shaped light source 101 emits light in a predetermined amount.
As the mirror 103-a is moved by a mechanism (not shown) in a main-scanning direction B at a scanning speed v, and the mirrors 103-b and 103-c move in a sub-scanning direction A at a speed of v/2 in synchronism with movement of the mirror 103-a. Thereby, the image representing an original surface of the main-scanning direction is sequentially formed on the line sensor 101 as a solid-state image sensing element. The image formed on the solid-state image sensing element 105 is converted into an electrical signal, sent to an output device (not shown), and printed, or sent to a storage device to store the input image information.
As the light source of such an image reading apparatus, a halogen lamp is conventionally used. A halogen lamp has a high luminance. However, since this lamp exhibits a large increase in temperature and requires power consumption of 200 to 300 W. power consumption of the entire apparatus increases. In recent years, to avoid this problem, high-luminance fluorescent lamps or xenon lamps have been developed and used as light sources for image reading apparatuses.
Generally, a fluorescent lamp or xenon lamp seals a small content of mercury mass and Ar or Kr, or Xe at several Torr in a rod-shaped hollow tube. Various phosphors are applied to the inner wall of the hollow tube, and electrodes are formed at the two ends of the hollow tube. In a fluorescent lamp or xenon lamp with this structure, UV rays are emitted from mercury or various gases upon discharge from the electrodes, and accordingly, the phosphors applied to the inner wall of the tube are excited to emit visible light in accordance with the light-emitting characteristics of the phosphors. Phosphors to be employed are selected in accordance with spectral energy characteristics required for a light source. Especially, a color image reading apparatus requires a light source having a wide wavelength range corresponding to R, G, and B (red, green, and blue) components. When a light source with a particularly high luminance is necessary, phosphors of a plurality of colors are mixed and applied to the inner wall of a tube.
However, the above-described conventional image reading apparatus has the following disadvantages.
The light-emitting amount (light-emitting intensity) of a fluorescent lamp or xenon lamp is generally controlled by pulse-width modulation (PWM) for controlling the pulse width corresponding to the ON time while keeping the value of a current flowing to the lamp constant, unlike a halogen lamp which controls the lighting voltage. PWM is employed because a fluorescent lamp or xenon lamp starts light emission when the current value exceeds a predetermined value. If the light-emitting amount is controlled by controlling the value of the current to be supplied, the range of light-emitting amount control becomes narrow.
FIG. 30 shows a control waveform for controlling the light-emitting amount of a fluorescent lamp by pulse-width modulation. In FIG. 30, the abscissa represents time, and the ordinate represents a current value for controlling the light-emitting amount of the light source. A period Hsync on the abscissa represents a time corresponding to a predetermined storage time of a solid-state image sensing element. This time corresponds to a time when charges are stored in correspondence with the amount of light incident on the light-receiving portion of the solid-state image sensing element.
For normal pulse width control, a control signal is output once per storage time in synchronism with the rise or fall of a trigger signal indicating the start of the period (period of time) Hsync as the storage time. When dimming is controlled in synchronism with a signal corresponding to the trigger signal of one storage time, noise due to a beat generated by interference between the storage time and pulse width control for controlling the light amount is removed from an image signal.
As a fluorescent lamp or xenon lamp coated with phosphors and used in an image reading apparatus for reading color image information, a white light source is often employed. In this light source, phosphors of a plurality of colors are mixed and applied to the inner wall of the lamp to simultaneously emit light components of various colors, thereby obtaining light-emitting characteristics in a wide wavelength range across the visible light range.
A white light source has a problem due to the difference in afterglow characteristics unique to the phosphors of different colors. Here, the afterglow characteristics mean that emitted light remains even after the current for controlling light emission of the light source is instantaneously cut off. Afterglow characteristics depend on the time when a phosphor excited by UV rays is staying at a high energy level and generally decrease as an exponential function. Depending on the characteristics of the material of a phosphor, the afterglow characteristics can be represented byT=e(τ−1)where τ represents characteristics determined by the material of a phosphor. When phosphors corresponding to R, G, and B colors are mixed, as in a white light source, τ changes in units of colors. A material used as a phosphor is generally determined on the basis of the light-emitting wavelength characteristics in a wavelength range, luminous efficiency, and service life of the material. Following materials are often used.
Blue: BaMg2Al16O27 (center wavelength 452 nm, T=2 μsec)
Red: Y2O3: Eu2+ (center wavelength 611 nm, T=1.1 msec)
Green: LaPO4: Ce, Tb (center wavelength 544 nm, T=2.6 msec)
T is the attenuation time of each material when the light-emitting amount reaches 1/e due to attenuation.
Since different colors have different afterglow characteristics (especially blue light has a short attenuation time), the barycenter of a read position in the sub-scanning direction changes depending on the color. This phenomenon will be described with reference to FIG. 31.
The abscissa of the graph shown in FIG. 31 represents time, and the ordinate represents the amount of a current for driving a fluorescent lamp and the light-emitting amount of the fluorescent lamp. FIG. 31 shows the model of afterglow generated on the basis of the attenuation characteristics of the R, G, and B colors.
Normally, light amount control (also called dimming control or dimming) of a fluorescent lamp is performed once in the period Hsync corresponding to one storage time of a solid-state image sensing element. The solid-state image sensing element stores charges in proportion to the amount of incident light.
In FIG. 31, the dimming period corresponds to a time when a current for driving the fluorescent lamp is supplied in an amount proportional to the dimming duty. As a technique mainly used, the current is switched to a high frequency during this period.
After the time corresponding to the dimming period, the light-emitting amount decreases. The attenuation characteristics are determined by the following two factors. One is the attenuation characteristics of a bright line spectrum generated by the fluorescent lamp, and the other is the above-described attenuation characteristics of the phosphor.
Normally, one storage time corresponding to the period Hsync is several hundred μsec. A bright line spectrum attenuates for 1 μsec or less and rarely influences. However, a phosphor attenuates on the order of millisecond and considerably influences. Hence, the attenuation characteristics of a light-emitting amount are determined by the sum of the light-emitting amounts of two types and the attenuation characteristics of each light emission.
In a fluorescent lamp turned on by a substantially predetermined current to emit light in a substantially predetermined amount during the dimming period, the light amount corresponding to the bright line spectrum instantaneously decreases when the dimming period is ended. This corresponds to a portion L1. In addition, afterglow corresponding to a portion L2 is generated due to the attenuation characteristics of the fluorescent lamp.
The afterglow characteristics of color light components have the following problem in an image reading apparatus.
One storage time of the solid-state image sensing element serves not only as a reference time in reading image information but also as a reference read position in reading in the sub-scanning direction.
The pixel density in reading image information is determined by the pixel size of the solid-state image sensing element in the main scanning direction, and the moving distance in image reading by mirror scanning in the sub-scanning direction.
Hence, the phenomenon that the light-emitting amounts of color light components have different barycenter positions with respect to the time Hsync because of their afterglow characteristics may be considered by replacing the abscissa of the graph in FIG. 31 with position information.
In FIG. 32, the abscissa is replaced with the distance in the sub-scanning direction. The barycenters (barycenter R and barycenter G) of the read positions of the R and G components in the sub-scanning direction move with respect to the barycenter position (barycenter B) of the B component with the smallest afterglow amount by d2. When the barycenter of the read position in the sub-scanning direction changes in units of colors, color misregistration occurs in reading in the sub-scanning direction to degrade the performance of the image reading apparatus. In an image obtained by reading a thin black line, a phase shift between the R, G, and B signals appears at the edge portion of the thin black line. If color misregistration occurs at the edge portion of the thin black line, the black thin line contained in the original cannot be expressed, and the image quality is degraded.
For an image reading apparatus using a fluorescent lamp or xenon lamp, another technique is examined in which the above-described light amount control is omitted, gain setting of an amplifier for electrically amplifying an output signal from the solid-state image sensing element is changed in accordance with a decrease in light amount due to durability, and an appropriate signal output is obtained by changing the gain in accordance with the decrease in light amount. However, when the gain is changed, the S/N ratio of the read signal varies depending on the value of the gain.